A capacitance detection apparatus is adapted to a triggering detection for an unlocking operation in a system for controlling an open/close of a door for a vehicle, such as an automobile. Specifically, when a user approaches the vehicle, an ID code certification is performed between the vehicle and a mobile system of the user. Then, the vehicle becomes a permission mode for unlocking. After that, when the user touches an unlocking sensor (electrode) provided in an outside handle of the door of the vehicle, the vehicle detects a change of a capacitance of the unlocking sensor electrode, and performs the unlocking operation. In other words, the capacitance detection apparatus detects a user's intention for unlocking by an output of the unlocking sensor (the change of the capacitance). On the other hand, when the user touches a locking switch provided in the outside handle at the time the user departs from the vehicle, the vehicle performs a locking operation after performing the ID certification between the vehicle and the mobile system of the user. A replacement of the locking switch with the capacitance detection typed sensor similar to the unlocking sensor is expected.
A known art relating to a capacitance detection apparatus is described in U.S. Pat. No. 3,886,447A. The capacitance detection apparatus detects a capacitance of a capacitor to be measured by charging the capacitor to be measured while both ends of the capacitor to be measured are connected to an output terminal and a GND terminal of a DC voltage, and by transferring the charge stored at the capacitor to be measured to a known standard capacitor while the both ends of the capacitor to be measured are connected to both ends of the standard capacitor by a switch, and by referring to a voltage of the standard capacitor determined by a ratio of the capacitance of the capacitor to be measured to the standard capacitor.
Further, the other known art relating to a capacitance detection apparatus is described in U.S. Pat. No. 4,039,940A. The capacitance detection apparatus obtains a capacitance of a capacitor to be measured by repeatedly performing an operation for charging an unknown capacitor to be measured by a DC voltage source and an operation for transferring the stored charge of the capacitor to be measured to a standard capacitor, and by repeatedly performing an operation for charging a known reference capacitor by the DC voltage source and an operation for transferring the stored charge of the reference capacitor to the standard capacitor, and by referring a ratio of the number of repetition of a charge transfer operation from the capacitor to be measured to the number of repetition of a charge transfer operation from the reference capacitor until a voltage of the standard capacitor becomes a predetermined voltage.
Further, the other known art relating to a capacitance detection apparatus is described in U.S. Pat. No. 6,466,036A. The capacitance detection apparatus includes a DC voltage source connected to one end of a standard capacitor via an open/close switch S1, a capacitor to be measured, one end of the capacitor to be measured is connected to the other end of the standard capacitor via a sensor electrode, the other end of the capacitor to be measured is connected to ground or free space, an open/close switch S2 provided between the other end of the standard capacitor and ground, an open/close switch S3 provided between both ends of the standard capacitor, and a voltage measurement portion for measuring a voltage of one end of the standard capacitor. The capacitance detection apparatus detects a change of the capacitance of the capacitor to be measured by repeating following operation. First, the switch S2 and the switch S3 are closed to discharge the standard capacitor and the capacitor to be measured. Next, the switch S1 is closed to charge the standard capacitor and the capacitor to be measured by the DC voltage source. The voltage of the standard capacitor is raised to a voltage determined by a ratio of the capacitor to be measured to the standard capacitor. After that, the switch S1 is opened. Further, the switch S2 is closed to connect the other end of the standard capacitor to ground for discharging the capacitor to be measured. Finally, the voltage of the standard capacitor is measured by the voltage measurement portion. These operations are repeated until the voltage of the standard capacitor is raised to the predetermined voltage, and the number of repetition of the operations is counted. Then, by detecting increase or decrease of the number of repetition, the capacitance detection apparatus detects the change of the capacitance of the capacitor to be measured.
Further, the known art in U.S. Pat. No. 6,466,036A describes the capacitance detection apparatus including two capacitors to be measured, two voltage measurement portions provided at both sides of the standard capacitor symmetrically for measuring voltage between the open/close switch S1, the open/close switch S2, and both terminals of the standard capacitor. The respective one ends of the capacitors to be measured are connected to both ends of the standard capacitor via the sensor electrodes respectively. The other ends of the capacitors to be measured are connected to ground or free space. The capacitance detection apparatus detects the changes of the capacitances of the two capacitors to be measured by the detection circuit described above.
According to the known art described in U.S. Pat. No. 3,886,447A, the both ends of the capacitor to be measured should be switched by the pair of the switch. Therefore, the capacitance detection apparatus described in U.S. Pat. No. 3,886,447A can not be adapted to the unlocking sensor provided in the outside handle for the vehicle door having the sensor electrode and the other end of the capacitor to be measured is connected to ground or a low impedance portion such as free space equivalent to ground. Further, in case that there are a leak resistance parallel to the capacitor to be measured, in other words, the unlock sensor electrode is in highly humid condition or wet with water, the capacitor to be measured is discharged except when the both ends of the capacitor to be measured are connected to the DC voltage source, and the standard capacitor is discharged when the both ends of the capacitor to be measured are connected to the standard capacitor, which causes a sensitivity lowering.
According to the known art described in U.S. Pat. No. 4,039,940A, since only ground side terminal of the capacitor to be measured is switched, the capacitance detection apparatus can be adapted to the detection of the capacitor to be measured, the one side of the capacitor to be measured is connected to ground via the one sensor electrode. However, in case that there is the leak resistance parallel to the capacitor to be measured, the standard capacitor and the capacitor to be measured are discharged, which causes detection sensitivity lowering, similar to the known art described in U.S. Pat. No. 3,886,447A.
According to the known art described in U.S. Pat. No. 6,466,036A, a terminal used as a standard (the other end of the standard capacitor) when measuring the voltage of the standard capacitor is connected to the sensor electrode and changed largely. Then, the voltage of the sensor electrode side terminal should be kept and stabilized at ground voltage by closing the switch S2 while measuring the voltage of the standard capacitor. As a result, the voltage measurement of the standard capacitor becomes complex and the measured voltage becomes unstable. Further, two respective sensors for locking and for unlocking connected to ground in the narrow space such as the outside handle induces a parasitic capacitance between the both electrodes. Therefore, in the capacitance detection apparatus described in U.S. Pat. No. 6,466,036A, the capacitance of the one sensor electrode is influenced by the other sensor electrode being touched by the human body through the parasitic capacitance, which causes an error detection of the trigger of the locking.
According to the known arts described in U.S. Pat. No. 3,886,447A and U.S. Pat. No. 4,039,940A, after charging the capacitor to be measured by connecting the DC voltage source to the capacitor to be measured, the charge at the capacitor to be measured is transferred to the standard capacitor by disconnecting the capacitor to be measured from the DC voltage source, and by connecting the standard capacitor in parallel to the capacitor to be measured, which raises the voltage of the standard capacitor (so called a parallel circuit type). According to the known art described in U.S. Pat. No. 6,466,036A, the voltage of the standard capacitor is raised by applying the voltage from the DC voltage source to the standard capacitor and the capacitor to be measured connected in series, and charging the standard capacitor and the capacitor to be measured simultaneously (so called a series circuit type). The series circuit typed detection circuit has advantage over the parallel circuit typed detection circuit in regard to a large voltage increase ratio (sensitivity) of the standard capacitor by transferred charge transferred from the capacitor to be measured.
A need thus exists a capacitance detection apparatus with the series circuit typed detection circuit, which enables to measure the voltage of the standard capacitor with stability by simple structure. Further need thus exists a capacitance detection apparatus, which reduces an influence from the leak resistance causing sensitivity lowering even when there is the leak resistance at the capacitor to be measured. Further need thus exists a capacitance detection apparatus, which makes possible to prevent the detection error by detecting one of the two capacitors to be measured connected electrostatically each other through the parasitic capacitance without the influence from the other capacitor through the parasitic capacitance.